This application relates generally to high-frequency and broadband device and module structures (e.g., modules for photonics and telecommunications applications), and more specifically to substrate structures that support the components during operation.
There is typically a trade-off between the performance of a device and the ease of manufacture of such device, and the higher cost of better-performing products has been accepted as an immutable law of nature. Photonic modules present an example of the trade-off between performance and manufacturability.
Current photonic modules are hybrid assemblies containing various optical, mechanical, and electronic parts. In many instances, the assembly considerations and the operational considerations for a given component may be incompatible with each other, and further, the assembly and operational considerations for one component may be incompatible with those for another. The substrate of a hybrid photonic assembly is a particular example of such competing considerations.
For example, in many instances soldering is the preferred bonding technique, and electrical resist soldering is one practical technique. In electrical resist soldering, a resistor pad is formed on the substrate at or near the bond site, and current is delivered to the resistor to provide the necessary heating. This militates toward a substrate with a low thermal conductivity so that the heat generated in the resistor stays localized to achieve required temperature. At the same time, the substrate is often required to provide heat dissipation and thermal equalization, which militates towards a substrate with a high thermal conductivity. In addition, the soldering of a fiber is carried out close to an optical coating on the front facet of the laser device, which can be damaged by excessive heat. Further complicating this in the case of a modulated laser, for example, is the fact that the substrate needs to provide for transmission lines that deliver RF signals to components attached to this substrate.
Previous approaches to the problem included cutting the substrate to provide a heat-blocking slot between the laser and the fiber solder site nearest the laser, or mounting the laser on a separate substrate. Forming a slot in the substrate, while effective to block heat from the laser, can lead to mechanical instabilities in the module. Mounting the laser on a separate substrate complicates the assembly fabrication and is also subject to mechanical instabilities.